Image processing method and apparatus

ABSTRACT

An image processing method includes the steps of performing resolution conversion processing on input image data to increase the number of grayscale levels, performing edge enhancement processing to increase the number of grayscale levels of the input image data, and performing dithering processing to reduce the number of grayscale levels of each pixel of the input image data. The number of pseudo grayscale levels is determined, based on a combination of a parameter concerning the resolution conversion processing and a parameter indicating the level of the edge enhancement processing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to image processing methods andapparatuses, and more particularly, to a method for processing digitalimage signals.

2. Description of the Related Art

Thin display devices, liquid crystal displays (LCDs), plasma displays(PDPs), and field emission displays (FEDs) have attracted attention.

The LCDs, PDPs, and FEDs are fixed-pixel matrix-driven display devices,which can be driven by digital image signals. The number of grayscalelevels of the above-described display devices is represented by thenumber of bits of a video signal corresponding to each pixel.

Techniques for displaying images so that they can be visually aestheticto the human eye by performing signal processing on image signals arebeing considered. Such techniques include edge enhancement processingfor enhancing edge portions and high-frequency components of images soas to apparently increase the resolution of the images.

FIG. 8 illustrates the configuration of an edge enhancer 800 forperforming edge enhancement processing on image signals.

An image signal input from an input terminal 801 is output to ahigh-pass filter 803 and also to an adder 807.

The high-pass filter 803 extracts high-frequency components of the inputimage signal and outputs the resulting image signal to a multiplier 805.

Under the control of a controller 809, the multiplier 805 multiplies anenhancement coefficient, indicating the level of enhancement of thehigh-frequency components of the image, by the high-frequencycomponents, and outputs the resulting signal to the adder 807. Bycontrolling the enhancement coefficient, the level of enhancement of thehigh-frequency components of the image can be adjusted.

As the bit precision of the high-frequency components which are outputfrom the multiplier 805, an 8-bit image signal input from the inputterminal 801 can be increased to a 12-bit image signal by the high-passfilter 803 and by the multiplier 805.

Then, the adder 807 adds the original 8-bit image signal and the 12-bithigh-frequency components output from the multiplier 805 and outputs theresulting 12-bit image signal having enhanced high-frequency componentsto a rounding unit 811.

By outputting an enhancement coefficient having a negative sign from thecontroller 809, the image can be made smoother instead of enhancing theedges.

To convert the 12-bit image signal into an 8-bit image signal, therounding unit 811 truncates the lower four bits of the 12-bit imagesignal by a rounding operation, and outputs the resulting 8-bit imagesignal to an output terminal 813.

FIG. 9 illustrates the configuration of an image processing apparatusfor performing resolution conversion on an image signal beforeperforming edge enhancement.

An image signal input from an input terminal 901 is supplied to aresolution converter 903.

The resolution converter 903 converts the resolution of the input signalto the resolution of a display device (not shown) under the control of acontroller 909.

If, for example, the horizontal resolution and the vertical resolutionof the display device are 1280 pixels and 720 pixels, respectively, andif the number of horizontal pixels and the number of vertical pixels ofan input image signal are 720 and 480, respectively, the horizontalresolution and the vertical resolution are scaled up by {fraction(16/9)} and {fraction (3/2)}, respectively.

If, for example, the horizontal resolution and the vertical resolutionof the display device are 1280 pixels and 720 pixels, respectively, andif the number of horizontal pixels and the number of vertical pixels ofan input image signal are 1920 and 1080, respectively, the horizontalresolution and the vertical resolution are scaled down by ⅔ and ⅔,respectively.

If the input pixel signal has 8 bits, the resolution converter 903expands the pixel signal into a 10-bit pixel signal to maintain theprecision of the converted pixel signal, and outputs the 10-bit pixelsignal to an edge enhancer 905.

Under the control of the controller 909, the edge enhancer 905 enhanceshigh-frequency components of the image and outputs the resulting imagesignal to a rounding unit 907.

If the input pixel signal has 10 bits, the edge enhancer 905 expands thepixel signal into a 12-bit pixel signal to maintain the precision of theimage signal with enhanced edges, and outputs the 12-bit pixel signal tothe rounding unit 907.

To reduce the 12-bit pixel signal into an 8-bit pixel signal, therounding unit 907 truncates the lower four bits of the pixel signal, andoutputs the resulting 8-bit data to an output terminal 911.

Meanwhile, the controller 909 controls the resolution conversion(scaling) ratio in the resolution converter 903 and also controls thelevel of edge enhancement in the edge enhancer 905.

In printers, halftone processing using dithering processing has beenperformed as a binarizing method. In printers, such as that disclosed inJapanese Patent Laid-Open No. 2000-134471, images are divided into, forexample, a character portion and a photograph portion, and differentbinarizing methods are used for these portions.

Japanese Patent Laid-Open No. 2003-69830 discloses an image processingmethod for performing resolution conversion and dithering processing.

When edge enhancement is conducted after performing resolutionconversion and when the resulting image is then rounded, pseudo contoursmay easily occur depending on the type of image.

When enlargement processing is performed, the correlation of adjacentpixels becomes high, and pseudo contours easily occur, which is visuallynoticeable.

To prevent the occurrence of pseudo contours, dithering processing canalways be performed instead of the rounding operation. In this case,however, dithering processing does not produce a noticeable effect onimages subjected to resolution reduction processing or edge enhancement,since pseudo contours do not easily occur because of the low correlationbetween adjacent pixels of such images. Conversely, dithering processingeasily produces an adverse influence, for example, noise having a fixedpattern, which is noticeable.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animage processing method and apparatus in which high quality images canbe displayed while effectively suppressing the occurrence of pseudocontours.

In order to achieve the above-described object, according to one aspectof the present invention, there is provided an image processing methodincluding the steps of: performing resolution conversion processing oninput image data to increase the number of grayscale levels; performingedge enhancement processing to increase the number of grayscale levelsof the input image data; performing dithering processing to reduce thenumber of grayscale levels of each pixel of the input image data; anddetermining the number of pseudo grayscale levels based on a combinationof a parameter concerning the resolution conversion processing and aparameter indicating the level of the edge enhancement processing.

According to another aspect of the present invention, there is providedan image processing method including the steps of: performing resolutionconversion processing on input image data to increase the number ofgrayscale levels; performing edge enhancement processing to increase thenumber of grayscale levels of the input image data; performing ditheringprocessing to reduce the number of grayscale levels of each pixel of theinput image data; and determining the number of pseudo grayscale levels,based on a combination of a parameter concerning the resolutionconversion processing and a parameter indicating the level of the edgeenhancement processing, so that the number of pseudo grayscale levels inthe dithering processing is increased when the resolution conversionprocessing is enlargement processing or when the level of the edgeenhancement processing is relatively low.

According to a further aspect of the present invention, there isprovided an image processing apparatus including: a resolution converterfor performing resolution conversion processing on input image data toincrease the number of grayscale levels; an edge enhancer for performingedge enhancement processing to increase the number of grayscale levelsof the input image data; and a dithering unit for performing ditheringprocessing to reduce the number of grayscale levels of each pixel of theinput image data. The number of pseudo grayscale levels is determinedbased on a combination of parameters concerning the resolutionconversion processing and the level of the edge enhancement processing.

According to the present invention, the number of pseudo grayscalelevels in dithering processing is controlled according to whether theresolution conversion processing is enlargement or reduction processingand the level of edge enhancement. It is thus possible to perform signalprocessing so that high quality images can be displayed whileeffectively inhibiting pseudo contours.

More specifically, when the resolution conversion processing isenlargement processing, the number of pseudo grayscale levels indithering processing is determined to be greater. When the level of edgeenhancement is relatively low, the number of pseudo grayscale levels indithering processing is determined to be greater. Accordingly, highquality images can be displayed while effectively inhibiting pseudocontours.

Further objects, features and advantages of the present invention willbecome apparent from the following description of the preferredembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example of the configurationof an image processing apparatus using an image processing methodaccording to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating the configuration of a ditheringunit.

FIGS. 3A through 3E illustrate threshold matrixes used in the ditheringunit.

FIGS. 4A and 4B illustrate tables indicating the correlation ofparameters used in an image processing method according to an embodimentof the present invention.

FIG. 5 is a block diagram illustrating another example of theconfiguration of an image processing apparatus using an image processingmethod according to another embodiment of the present invention.

FIG. 6 illustrates a table indicating an example of the correlation ofparameters used in an image processing method according to anotherembodiment of the present invention.

FIG. 7 illustrates a table indicating another example of the correlationof parameters used in the image processing method according to anotherembodiment of the present invention.

FIG. 8 is a block diagram illustrating an example of a known edgeenhancer.

FIG. 9 is a block diagram illustrating an example of a known imageprocessing apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described in detail below with reference to theaccompanying drawings through illustration of preferred embodiments.

First Embodiment

Referring to an image processing apparatus shown in FIG. 1, a digitalimage signal is input into an input terminal 1. In this embodiment, theinput digital pixel signal is quantized with eight bits.

The digital image signal input into the input terminal 1 is then outputto a resolution converter 4.

The resolution converter 4 converts the resolution of the input imagesignal to the resolution of a display device (not shown) under thecontrol of a controller 11. That is, by decreasing (reducing) orincreasing (enlarging) the number of pixels of an input image signal,the resolution of the input signal is converted into the number ofpixels of a display device or the number of pixels of a display area,such as a small window, in the display device. In this specification,the reduction/enlargement ratios are collectively referred to as the“resolution conversion (scaling) ratio”.

If, for example, the horizontal resolution and the vertical resolutionof the display device are 1280 pixels and 720 pixels, respectively, andif the number of horizontal pixels and the number of vertical pixels ofan input image signal are 720 and 480, respectively, the horizontalresolution and the vertical resolution are scaled up by {fraction(16/9)} and {fraction (3/2)}, respectively.

Although the type of enlargement processing used in the presentinvention is not restricted, interpolation methods, for example, linearinterpolation such as bilinear interpolation, or three-dimensionalconvolutional interpolation such as bicubic interpolation, arepreferable.

If, for example, the horizontal resolution and the vertical resolutionof the display device are 1280 pixels and 720 pixels, respectively, andif the number of horizontal pixels and the number of vertical pixels ofan input image signal are 1920 and 1080, respectively, the horizontalresolution and the vertical resolution are scaled down by ⅔ and ⅔,respectively.

As the reduction processing used in the present invention, pixel signalscan be simply eliminated, or after conducting coordinate transformationby the interpolation method such as linear interpolation orthree-dimensional convolutional interpolation, pixel signals atunnecessary coordinates can be eliminated.

When the input pixel signal has 8 bits, the resolution converter 4expands the 8-bit pixel signal into a 10-bit pixel signal to maintainthe precision of the converted image signal, and outputs the 10-bitpixel signal to an edge enhancer 5.

The edge enhancer 5 enhances high-frequency components of the imageunder the control of the controller 11, and outputs the resulting imageto a dithering unit 7. The controller 11 receives a parameter concerningthe resolution conversion and a parameter indicating the level of edgeenhancement as signals, and then determines the number of pseudograyscale levels in performing dithering processing based on acombination of the above-described parameters. In the edge enhancer 5,in order to maintain the computation precision increased by edgeenhancement, the 10-bit pixel signal is increased to a 12-bit pixelsignal.

FIG. 2 illustrates the configuration of the dithering unit 7 shown inFIG. 1.

The 12-bit pixel signal output from the edge enhancer 5 shown in FIG. 1is input into an adder 25 shown in FIG. 2.

Meanwhile, a threshold matrix 23 outputs a threshold matrix indicated byone of FIGS. 3A through 3E to the adder 25 according to the position ofthe pixel. These threshold matrixes are each formed of a memory or aregister, and can be rewritten by the controller 11.

The adder 25 adds the 12-bit pixel image and the threshold of the 4-bitdither matrix, and outputs the added value to a divider 27.

If the most significant bit (MSB) is carried to a higher digit as aresult of adding the 12-bit pixel signal and the 4-bit threshold data,the resulting value may be converted into 12 bits by performing clippingbefore being output to the divider 27.

Clipping replaces the bit length of a pixel signal in excess of a presetmaximum value by the maximum value.

The divider 27 divides the input 12-bit pixel signal so as to reduce itinto an 8-bit pixel signal, and outputs the resulting signal to anoutput terminal 13 shown in FIG. 1.

The divider 27 performs a dividing operation by truncating a pixelsignal. For example, when the pixel signal output from the adder 25 is apixel signal clipped to 12 bits, the divider 27 may shift the signal byfour bits.

The rounding operation of the pixel signal by using a dither matrix isdiscussed below.

Only the MSBs of the binary digital data of the matrix are set to 1.That is, if the matrix is a 4-bit threshold matrix, all the columns ofthe matrix are set to 1000 in binary digital data, i.e., to 8 in decimalnotation, as shown in FIG. 3D. The adder 25 adds the 12-bit pixel signaland the threshold of the threshold matrix 23 and outputs the resultingvalue to the divider 27. Then, in the divider 27, the lower four bitsare truncated.

The controller 11 controls the resolution ratio in the resolutionconverter 4 and the level of edge enhancement in the edge enhancer 5,and also controls the threshold of the dither matrix in the ditheringunit 7 according to whether the resolution processing is enlargement orreduction processing and the level of edge enhancement in the edgeenhancer 5.

In the threshold matrix shown in FIG. 3A, the bit length is four bits,and the number of bits which can represent the number of grayscalelevels in a pseudo manner is four bits, or 0 to 15 (16 levels). The sizeof the matrix is 4×4.

In the threshold matrix shown in FIG. 3B, although the bit length isfour bits, the least significant bit (LSB) is 0 and the number of bitswhich can represent the number of grayscale levels in a pseudo manner is3 bits, i.e., 0, 2, 4, 6, 8, 10, 12, and 14 (8 levels).

In the threshold matrix shown in FIG. 3C, although the bit length isfour bits, the lower two bits are 0 and the number of bits which canrepresent the number of grayscale levels in a pseudo manner is 2 bits,i.e., 0, 4, 8, and 12 (4 levels).

FIGS. 4A and 4B illustrate correlation tables when the threshold of thedither matrix is controlled according to whether enlargement orreduction processing is performed and the level of edge enhancement. Thelevel of edge enhancement can be changed by increasing or decreasing thecoefficient to be multiplied with high-frequency components in themultiplier, as stated above.

FIG. 4A illustrates a mode in which, when the resolution conversion isenlargement processing, priority is given to the resolution conversionover edge enhancement, and pseudo contours are prevented by increasingthe number of pseudo grayscale levels by dithering.

In FIG. 4A, when the resolution conversion is enlargement processing andedge enhancement is performed at a low level (mode 31), the resultingimage becomes the smoothest, and if bits of the resulting signal aretruncated after performing edge enhancement, it is most likely thatpseudo contours occur. Thus, in mode 31, the number of pseudo grayscalebits is set to be four, i.e., the dither matrix shown in FIG. 3A isused.

In this case, the number of grayscale levels that can be apparentlyrepresented is a total of 12 bits (the number of grayscale levels (8bits) of the original pixel signal and the number of pseudo grayscalelevels (4 bits) by dithering processing).

In FIG. 4A, when the resolution conversion is enlargement processing andedge enhancement is performed at a high level (mode 32), the resultingimage becomes the second smoothest, and it is relatively likely thatpseudo contours occur. Thus, in mode 32, the number of pseudo grayscalebits is set to be three, i.e., the dither matrix shown in FIG. 3B isused.

In this case, the number of grayscale levels that can be apparentlyrepresented is a total of 11 bits (the number of grayscale levels (8bits) of the original pixel signal and the number of pseudo grayscalelevels (3 bits) by dithering processing).

In FIG. 4A, when the resolution conversion is reduction processing andedge enhancement is performed at a low level (mode 33), it is relativelyunlikely that pseudo contours occur. Thus, in mode 33, the number ofpseudo grayscale bits is set to be two, i.e., the dither matrix shown inFIG. 3C is used.

In this case, the number of grayscale levels that can be apparentlyrepresented is a total of 10 bits (the number of grayscale levels (8bits) of the original pixel signal and the number of pseudo grayscalelevels (2 bits) by dithering processing).

In FIG. 4A, when the resolution conversion is reduction processing andedge enhancement is performed at a high level (mode 34), it is leastlikely that pseudo contours occur, and even if pseudo contours occur,they are unnoticeable. Thus, in mode 34, the image signal is roundedwithout performing dithering processing.

In this case, the rounding operation may be performed as follows. Allthe threshold levels in the dither matrix are fixed to 8, as shown inFIG. 3D, and the resulting threshold is added to the 12-bit pixelsignal, and then, the lower four bits of the resulting value aretruncated.

In this case, the number of grayscale levels that can be apparentlyrepresented is a total of 8 bits.

FIG. 4B illustrates a mode in which, when edge enhancement is performedat a low level, priority is given to edge enhancement over theresolution conversion (enlargement processing), and pseudo contours areprevented by increasing the number of pseudo grayscale levels that canbe represented by dithering.

In FIG. 4B, mode 41 is similar to mode 31 in FIG. 4A, and similarly,mode 44 is similar to mode 34 in FIG. 4A.

In FIG. 4B, when edge enhancement is performed at a low level and theresolution conversion is reduction processing (mode 42), the number ofpseudo grayscale levels represented by dithering is set to be three,i.e., the dither matrix shown in FIG. 3B is used.

In this case, the number of grayscale levels that can be apparentlyrepresented is a total of 11 bits (the number of grayscale levels (8bits) of the original pixel signal and the number of pseudo grayscalelevels (3 bits) by dithering processing).

In FIG. 4B, when edge enhancement is performed at a high level and theresolution conversion is enlargement processing (mode 43), the number ofpseudo grayscale levels represented by dithering is set to be two, i.e.,the dither matrix shown in FIG. 3C is used.

In this case, the number of grayscale levels that can be apparentlyrepresented is a total of 10 bits (the number of grayscale levels (8bits) of the original pixel signal and the number of pseudo grayscalelevels (2 bits) by dithering processing).

The image processing apparatus of this embodiment is connected to afixed-pixel matrix-driven display device via the output terminal 13, ifnecessary, through a signal processing circuit or a drive circuit, andthe processed image data is supplied to and displayed on the displaydevice.

Second Embodiment

FIG. 5 is a block diagram illustrating another example of aconfiguration of an image processing apparatus using an image processingmethod according to another embodiment of the present invention.

A digital image signal is input into an input terminal 501. In thisembodiment, an input digital pixel signal is quantized with eight bits.

The digital pixel signal input into the input terminal 501 is output toa delay unit 503 and a dynamic range detector 511.

In the delay unit 503, the digital image signal is delayed until thedynamic range of the image signal is detected in the dynamic rangedetector 511. For example, to detect the dynamic range of one frame of atelevision signal, the delay unit 503 delays the television signal byone frame. The delayed digital image signal is output to a resolutionconverter 505.

The resolution converter 505 converts the resolution of the input signalto the resolution of a display device, such as the number of pixels of afixed-pixel display device (not shown).

If, for example, the horizontal resolution and the vertical resolutionof the display device are 1280 pixels and 720 pixels, respectively, andif the number of horizontal pixels and the number of vertical pixels ofan input image signal are 720 and 480, respectively, the horizontalresolution and the vertical resolution are scaled up by {fraction(16/9)} and {fraction (3/2)}, respectively.

Although the type of enlargement processing used in the presentinvention is not restricted, interpolation methods, for example, linearinterpolation such as bilinear interpolation, or three-dimensionalconvolutional interpolation such as bicubic interpolation, arepreferable.

If, for example, the horizontal resolution and the vertical resolutionof the display device are 1280 pixels and 720 pixels, respectively, andif the number of horizontal pixels and the number of vertical pixels ofan input image signal are 1920 and 1080, respectively, the horizontalresolution and the vertical resolution are scaled down by ⅔ and ⅔,respectively. In this case, an 8-bit image is expanded into 10 bits.

As reduction processing is used in the present invention, pixel signalscan be simply eliminated, or after conducting coordinate transformationby interpolation methods such as linear interpolation orthree-dimensional convolutional interpolation, pixel signals atunnecessary coordinates can be eliminated.

An edge enhancer 507 enhances edge portions of the image under thecontrol of a controller 513 and outputs the resulting image to adithering unit 509. In the edge enhancer 507, in order to maintain thecomputation precision of the bits increased by the edge enhancement, the10-bit input pixel signal is increased to a 12-bit pixel signal.

Meanwhile, the dynamic range detector 511 detects the dynamic range ofone frame of the image signal and outputs the level of the dynamic rangeto the controller 513.

The controller 513 sets the enlargement/reduction ratios used in theresolution converter 505, and also controls the level of edgeenhancement in the edge enhancer 507 so as to control the threshold of adither matrix in the dithering unit 509 based on the level of thedynamic range input from the dynamic range detector 511, the level ofedge enhancement in the edge enhancer 507, and the enlargement orreduction ratio, i.e., the scaling ratio, used in the resolutionconverter 505.

As the thresholds of the dither matrixes, the same matrixes as thoseshown in FIGS. 3A through 3E can be used.

Although the bits of the threshold matrix shown in FIG. 3E is four bits,the lower three bits are 0 and the number of bits which can representthe number of grayscale levels in a pseudo manner is 1 bit, i.e., 0 and8 (2 levels).

FIG. 6 illustrates a correlation table when the threshold of the dithermatrix is controlled according to the type of resolution conversion(i.e., enlargement or reduction), the level of the dynamic range, andthe level of edge enhancement.

In the correlation table shown in FIG. 6, priority is given toenlargement processing of the resolution conversion over the level ofedge enhancement, and pseudo contours are prevented by increasing thenumber of pseudo grayscale levels by dithering processing.

In FIG. 6, when the dynamic range of the image signal is narrow, whenthe resolution conversion is enlargement processing, and when edgeenhancement is performed at a low level (mode 11), the resulting imagebecomes the smoothest, and it is most likely that pseudo contours occur.Thus, in mode 11, the number of pseudo grayscale bits represented bydithering is set to be four, i.e., the dither matrix shown in FIG. 3A isused.

In this case, the number of grayscale levels that can be apparentlyrepresented are a total of 12 bits (the number of grayscale levels (8bits) of the original pixel signal and the number of pseudo grayscalelevels (4 bits) by dithering processing).

In FIG. 6, when the dynamic range of the image signal is wide, when theresolution conversion is enlargement processing, and when edgeenhancement is performed at a low level (mode 12), the resulting imagebecomes the second smoothest. Thus, in mode 12, the number of pseudograyscale bits represented by dithering is set to be three, i.e., thedither matrix shown in FIG. 3B is used.

In this case, the number of grayscale levels that can be apparentlyrepresented is a total of 11 bits (the number of grayscale levels (8bits) of the original pixel signal and the number of pseudo grayscalelevels (3 bits) by dithering processing).

In FIG. 6, when the dynamic range of the image signal is narrow, whenthe resolution conversion is reduction processing, and when edgeenhancement is performed at a low level (mode 15), pseudo contourseasily occur if the dynamic range of the image is an intermediate level.

Thus, in mode 15, the number of pseudo grayscale bits represented bydithering is set to be two, i.e., the dither matrix shown in FIG. 3C isused.

In this case, the number of grayscale levels that can be apparentlyrepresented is a total of 10 bits (the number of grayscale levels (8bits) of the original pixel signal and the number of pseudo grayscalelevels (2 bits) by dithering processing).

In FIG. 6, when the dynamic range of the image signal is narrow, whenthe resolution conversion is reduction processing, and when edgeenhancement is performed at a high level (mode 17), it is less likelythat pseudo contours occur.

Thus, in mode 17, the number of pseudo grayscale bits represented bydithering is set to be one, i.e., the dither matrix shown in FIG. 3E isused.

In this case, the number of grayscale levels that can be apparentlyrepresented is a total of 9 bits (the number of grayscale levels (8bits) of the original pixel signal and the number of pseudo grayscalelevel (1 bit) by dithering processing).

In FIG. 6, when the dynamic range of the image signal is wide, when theresolution conversion is reduction processing, and when edge enhancementis performed at a high level (mode 18), it is least likely that pseudocontours occur. Accordingly, the resulting image is truncated withoutperforming dithering processing.

In this case, the rounding operation may be performed as follows. Allthe threshold levels in the dither matrix are fixed to 8, as shown inFIG. 3D, and the resulting threshold is added to the 12-bit pixelsignal, and then, the lower four bits of the resulting value aretruncated.

In this case, the number of grayscale levels that can be apparentlyrepresented is a total of 8 bits.

FIG. 7 illustrates a correlation table in which priority is given to thelevel of edge enhancement over enlargement/reduction processing of theresolution conversion, and pseudo contours are prevented by increasingthe number of pseudo grayscale levels by dithering processing.

In FIG. 7, mode 21 is similar to mode 11 in FIG. 6. Similarly, mode 22,mode 27, and mode 28 are similar to mode 12, mode 17, and mode 18,respectively, in FIG. 6.

In mode 23, however, the number of pseudo grayscale bits represented bydithering processing is greater than that in mode 15 shown in FIG. 6.Likewise, in mode 24, the number of pseudo grayscale bits represented bydithering processing is greater than that in mode 16 shown in FIG. 6. Inmode 25, the number of pseudo grayscale bits represented by ditheringprocessing is smaller than that in mode 13 shown in FIG. 6. In mode 26,the number of pseudo grayscale bits represented by dithering processingis smaller than that in mode 14 shown in FIG. 6.

The processing modes in FIG. 6 are preferable when theenlargement/reduction (scaling) ratio of the resolution conversion islarge, while the processing modes in FIG. 7 are preferable when theenlargement/reduction (scaling) ratio of the resolution conversion issmall.

In FIG. 7, when the edge enhancement is performed at a low level, whenthe resolution conversion is reduction processing, and when the dynamicrange of the image signal is wide (mode 24), pseudo contours likelyoccur if the dynamic range of the image is an intermediate level.

Thus, in mode 24, the number of pseudo grayscale bits represented bydithering is set to be two, i.e., the dither matrix shown in FIG. 3C isused.

In this case, the number of grayscale levels that can be apparentlyrepresented is a total of 10 bits (the number of grayscale levels (8bits) of the original pixel signal and the number of pseudo grayscalelevels (2 bits) by dithering processing).

The image processing apparatus of this embodiment is connected to afixed-pixel matrix-driven display device via an output terminal 515, ifnecessary, through a signal processing circuit or a drive circuit, andthe processed image data is supplied to and displayed on the displaydevice.

According to the above-described embodiments, a digital pixel signalhaving real grayscale levels (without pseudo grayscale levels) subjectedto image processing is input into a modulation drive circuit of afixed-pixel matrix-driven display. The digital pixel signal is thensubjected to pulse width modulation, voltage amplitude modulation, orcurrent amplitude modulation, or a combination of pulse width modulationand voltage amplitude modulation (or current amplitude modulation). Theresulting modulated output signal is then supplied to the correspondingpixel. The luminance of the pixel is exhibited with the real grayscalelevels based on the modulated output signal. However, from the point ofthe entire image of one screen, since the number of pseudo grayscalelevels by dithering processing is added to the number of real grayscalelevels, the image can be played back and displayed on the basis of atotal of the number of real grayscale levels and the number of pseudograyscale levels.

The present invention can be preferably used in fixed-pixelmatrix-driven display devices, for example, electron beam fluorescentdisplays having pixels consisting of at least one electron beam elementand a fluorescent material, such as FEDs and surface conduction displays(SEDs), natural light displays, such as PDPs and electroluminescencedisplays (ELDs), and displays such as LCDs.

While the present invention has been described with reference to whatare presently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

This application claims priority from Japanese Patent Application No.2003-387876 filed Nov. 18, 2003, which is incorporated herein byreference.

1. An image processing method comprising the steps of: performingresolution conversion processing on input image data to increase thenumber of grayscale levels; performing edge enhancement processing toincrease the number of grayscale levels of the input image data;performing dithering processing to reduce the number of grayscale levelsof each pixel of the input image data; and determining the number ofpseudo grayscale levels based on a combination of a parameter concerningthe resolution conversion processing and a parameter indicating thelevel of the edge enhancement processing.
 2. An image processing methodcomprising the steps of: performing resolution conversion processing oninput image data to increase the number of grayscale levels; performingedge enhancement processing to increase the number of grayscale levelsof the input image data; performing dithering processing to reduce thenumber of grayscale levels of each pixel of the input image data; anddetermining the number of pseudo grayscale levels, based on acombination of a parameter concerning the resolution conversionprocessing and a parameter indicating the level of the edge enhancementprocessing, so that the number of pseudo grayscale levels in thedithering processing is increased when the resolution conversionprocessing is enlargement processing or when the level of the edgeenhancement processing is relatively low.
 3. The image processing methodaccording to claim 2, wherein, when determining the number of pseudograyscale levels, priorities are assigned to the parameter concerningthe resolution conversion processing and the parameter indicating thelevel of the edge enhancement processing, and the number of pseudograyscale levels in the dithering processing is determined based on theassigned priorities.
 4. An image processing apparatus comprising: aresolution converter for performing resolution conversion processing oninput image data to increase the number of grayscale levels; an edgeenhancer for performing edge enhancement processing to increase thenumber of grayscale levels of the input image data; and a dithering unitfor performing dithering processing to reduce the number of grayscalelevels of each pixel of the input image data, wherein the number ofpseudo grayscale levels is determined based on a combination ofparameters concerning the resolution conversion processing and the levelof the edge enhancement processing.
 5. The image processing apparatusaccording to claim 4, further comprising a fixed-pixel matrix-drivendisplay, wherein the image data subjected to image processing issupplied to and displayed on the fixed-pixel matrix-driven display.